Production and preservation of archaeal glycerol dibiphytanyl glycerol tetraethers as intact polar lipids in marine sediments: Implications for their use in microbial ecology and TEX86 paleothermometry

Abstract

For climate modelling in order to predict climate change scenarios, it is important to consider past climate conditions and their impacts. The study of past climates, paleoclimatology, makes frequent use of molecular fossils, molecules that are preserved over geological time scales. Ratios of different compounds can be used, for example, as paleothermometers in order to determine temperatures of air and sea millions of years ago. One of these paleothermometers is the so-called TEX86 which is based on compounds present in the membranes of microorganisms called Thaumarchaeota, that thrive in the ocean. Their membrane lipids are glycerol diether glycerol tetraethers with hydrophilic headgroups (IPL-GDGTs), which are degraded after cell death to core lipid (CL-) GDGTs by release of the head groups, and settle to the sea floor packaged in organic matter particles. After deposition, the distribution of the different GDGTs on which the TEX86 is based can potentially be altered through several process, thereby altering the temperature signal. Sedimentary Thaumarchaeota could produce GDGTs within the sediment, and degradation processes, especially exposure to oxygen, could degrade some GDGTs faster than others. The aim of this thesis was to investigate the effect of post-depositional processes in marine sediments have on the GDGT distribution and thus the TEX86. This was achieved by analysis of IPL- and CL-GDGT distributions, and by conducting stable isotope probing experiments. Results showed that GDGTs from pelagic Thaumarchaeota are deposited and preserved in the sediment as both IPL- and CL-GDGTs. This suggests that IPL-GDGTs are not representative for live Thaumarchaeota and can, dependent on their headgroups, be preserved over geological time scales. Furthermore, it was discovered that GDGT-distributions differ per type of head group, and preferential degradation of more labile head group-containing IPL-GDGTs can thus result in small distributional changes of the IPL-GDGTs. However, these changes are not large enough to change the TEX86 of CL-GDGTs, which is used in paleotemperature determinations. A lack of incorporation of 13C from different 13C-labelled substrates – phytodetritus, amino acids, glucose, pyruvate and bicarbonate – confirmed the slow turnover of IPL-GDGTs and indicated that, if sedimentary Thaumarchaeota are indeed active in sediments, they are only present in relatively low numbers and growing and metabolizing only slowly (up to 100s of years of doubling times). In situ produced GDGTs were shown to degrade completely rather than just lose their head groups and pass over into the CL-pool. Sedimentary processes thus hardly influence the TEX86. IPL-GDGT distributions can change with proceeding degradation, but CL-GDGT distributions are not affected by this. Activity and biomass of sedimentary Archaea have thus previously been overestimated, as the majority of IPL-GDGTs present in sediments are fossil